1. Field of the Invention
This invention relates to turbines for a combustion turbine power plant and, more specifically, to a secondary combustor which re-heats the working gas as the working gas passes through the turbine assembly.
2. Background Information
A conventional combustible gas turbine includes a compressor assembly, a combustor assembly, a transition section, and a turbine assembly. The compressor assembly compresses the ambient air. The combustor assembly combines the compressed air with a fuel and ignites the mixture creating a working gas. The working gas travels through the transition section to the turbine assembly. The turbine assembly is structured to have a cross-sectional area that increases as the working gas moves downstream. Also, within the turbine assembly are a series of rows of stationary vanes and rotating blades. Each pair of rows of vanes and blades is called a stage. Typically, there are four stages in a turbine assembly. The rotating blades are coupled to a shaft. As the working gas expands through the turbine assembly, the working gas causes the blades, and therefore the shaft, to rotate.
The power output for the turbine is proportional to the temperature of the working gas in the turbine assembly. That is, the higher the temperature of the working gas, the greater the power output of the turbine assembly. To ensure that the working gas has energy to transfer to the rotating blades within the final stage, the working gas must be at a high working temperature as the gas enters the turbine assembly. For modern engines, the working temperature, or turbine inlet temperature, is about 2800xc2x0 F. (1537xc2x0 C.). The temperature of the working gas is reduced. However, as the working gas passes through each stage of the turbine assembly. Thus, the power output generated from the later stages is less than optimal. The amount of power output for the later stages of the turbine assembly could be increased if the temperature of the working gas within the later stages was increased.
The easiest method of increasing the temperature of the working gas in the later stages of the turbine assembly is to increase the temperature of the working gas in the combustor assembly. Unfortunately, NOx is generally produced in high temperature (2650xc2x0 F./1455xc2x0 C. or greater) flame regions of the combustor assembly and the transition to the turbine assembly. The quantity of NOx produced increases as the temperature in the combustor increases. Additionally, increasing the operating temperature of the combustor increases the thermal stress on the components thereby requiring that the components be manufactured to withstand the higher temperatures. This adds to the manufacturing costs of the components.
Alternatively, discrete secondary combustors could be used. Unfortunately, undesirable amounts of NOx may be formed in such secondary combustors. This is because if the working gas spends a longer residence time at high ( greater than 2650xc2x0 F./1455xc2x0 C.) temperatures, NOx is formed by reaction of N2 and O2 in the gas.
There is, therefore, a need for a device to reheat the working gas within the turbine assembly but does not allow the working gas to remain at a high ( greater than 2650xc2x0 F./1455xc2x0 C.) temperature for a significant amount of time.
There is a further need for a working gas re-heating device having a low residence time flame.
There is a further need for a working gas re-heating device which is compatible with existing combustion turbines.
There is a further need for a working gas re-heating device which incorporates present technology used on combustion turbines.
These needs, and others, are satisfied by the present invention which provides a secondary combustor assembly within the stationary vanes and/or rotating blades within the turbine assembly. The secondary combustor is located within one or more stages of the turbine assembly and re-heats the working gas as the working gas passes through that stage of the turbine assembly.
The secondary combustor supplies combustible gas through the stationary vanes and/or the rotating blades in a turbine assembly. Many turbine assemblies presently include internal channels within the rotating blades and/or stationary vanes to allow a cooling gas, or steam, to pass therethrough. The channels may have openings to allow the cooling gas to join the working gas. The present invention provides a combustible gas, such as, but not limited to, natural gas, through the internal channels of the stationary vanes and/or rotating blades. As the combustible gas exits openings along the trailing edges of the stationary vanes and/or rotating blades, the combustible gas will spontaneously combust, or xe2x80x9cauto-ignite,xe2x80x9d upon being exposed to the heated working gas. The flame produced in the rotating blade and/or stationary vane portion of a turbine has a low residence time (typically 5 msec. or less), yet still provides enough energy to heat the working gas as the working gas passes through that stage of the turbine assembly.
The openings along the trailing edge have a diameter of less than about 0.125 inch. The high temperature flames extending from these openings are micro-diffusion flames. That is, the flames have small volumes, and therefore residence time of working gas within the flame region is very short. No significant amount of NOx is created in the rotating blade and/or stationary vane portion of the turbine due to the minimal amount of time the working gas is heated above 2650xc2x0 F./1455xc2x0 C.
One embodiment of the invention only has a secondary combustor located in the first row of vanes or blades. That is, the combustible gas is only channeled to the first row of vanes or blades. Another embodiment of the invention includes a secondary combustor in a plurality of the rows of vanes and/or blades. When the combustible gas is introduced into various points in the turbine assembly, the working gas is reheated in stages at each of these points. A third embodiment incorporates an additional stage in the turbine assembly. That is, the turbine assembly has five stages. In this embodiment, the compressor is structured to provide greater compression and the secondary combustor is located in the second stage of the turbine assembly.
The secondary combustor further provides advantage of having a primary combustion assembly which operates at a temperature approximately 500xc2x0 F.-300xc2x0 F. (260xc2x0 C.-149xc2x0 C.) lower than prior combustion turbines. A cost-savings can be realized by designing the combustor assembly and transition section to operate at the lower temperature. However, this device may also be used with current combustion turbine power plants which incorporate cooling passageways in the rotating blades and/or stationary vanes that are open to the working gas flow path.